1. Field of the Invention
The present invention relates to a lightning arrester provided with a gas-filled discharge tube for protecting electric appliances from a voltage surge caused by lightning or the like, and more particularly relates to a lightning arrester provided with a fail-safe system which is capable of protecting the appliances from damages even when the lightning arrester is subjected to a prolonged voltage surge and thus heated, and also provided with a vent-safe system capable of discharging a voltage surge to the ground, in place of the gas-filled discharge tube in event that the discharge tube loses its discharge capability.
2. Description of the Related Art
Gas-filled discharge tubes are incorporated in lightning arresters and function as overvoltage protectors, for protecting electric appliances from a voltage surge. Each gas-filled discharge tube comprises a cylindrical ground electrode, two insulation rings connected, at one end, to both ends of the ground electrode, respectively, and two main electrodes connected to the other ends of the insulation rings, respectively. To protect an electric appliance from a voltage surge, the ground electrodes are connected to the ground, and the main electrode is connected to two lines to which the electric appliance is connected.
When the lines are subjected to a voltage surge, the main electrode and the ground electrode cooperate, thus discharging the surge voltage to the ground, thereby protecting the electric appliance from the voltage surge.
When the voltage surge lasts short because it has resulted from lightning, it takes the discharge tube but a short time to discharge the surge voltage to the ground, and the tube is not overheated. When the voltage surge lasts long because it has resulted from the contact between the line and a power source line, it takes the tube a long time to discharge the surge voltage to the ground, and the tube is overheated. When the overheating occurs, the gas may leak from the discharge tube, in which case the discharge tube can no longer perform its function.
It is therefore required that a lightning arrester be provided with a fail- and vent-safe systems to prevent the gas-filled discharge tube from being disabled.
A lightning arrester disclosed in U.S. Pat. No. 4,150,414, for example, is provided with the fail- and vent-safe systems. A gas-contained discharge tube in this lightning arrester comprises a center metal cylinder (or ground electrode), insulation rings and a metal cap (or main electrode). A short-circuiting metal fitting comprising two clips and a coupling for connecting the two clips as a unit is attached to the gas-contained discharge tube by mean of the clips. The clips of the short-circuiting metal fitting is conductively connected to the metal cap. A heat-melted insulation sleeve is fitted onto the center cylinder and the short-circuiting metal fitting clips the center cylinder through the insulation sleeve. The short-circuiting metal fitting is thus insulated from the center cylinder while it is electrically connected to the metal cap. When the discharge tube is overheated because of long-time discharge and the insulation sleeve is thus melted, the clips of the short-circuiting metal fitting are contacted with the center cylinder due to their elasticity. The metal cap and the center cylinder are thus short-circuited through the short-circuiting metal fitting to thereby protect the lightning arrester and the electric appliances and machines connected to the lightning arrester from being abnormally overheated.
A metal rod is conductively attached to the center cylinder and the short-circuiting metal fitting is provided with a hole at a part thereof, which is closed by a collared metal member. An insulation member is sandwiched between the metal rod and the ceiling of the metal member to form an air gap therebetween. When the gas-contained discharge tube is broken and gas is thus leaked, therefore, discharge is carried out in the air gap to keep lightning arresting ability unchanged.
In the case of this lightning arrester disclosed by U.S. Pat. No. 4,150,414, however, the clips of the short-circuiting metal fitting must have a spring larger than a certain value. This makes the arm-like clips long and the whole of the device is made large in size accordingly. Further, the insulation member is interposed between the metal rod and the metal member to form the air gap therebetween. This makes the device complicated in structure and high in cost.